- pretty long and dry -
Very roughly, one can figure it takes about 1KV per .001" gap (assuming
a compressed gas/air environment) to reliably fire a spark plug. I know,
I know, it is much more complex than this (non-linear relationship,
compression ratio dependent, blah, blah, blah...), but this a reasonable
generalization.
Anyway, given this simple rule of thumb, if your coil can produce 40KV,
you should be able to run at least a .035" gap, against the coil. Now
as you open the gap, you of course, raise the voltage that the entire
secondary circuit (rotor, cap, spark plug wires, boots, etc.) must be
able to withstand (i.e., the gap will determine the voltage at which the
spark will occur--the greater the gap, the higher the voltage and the
longer the spark achieved). Likewise, if you spread the gap too wide,
you will not obtain a reliable spark (the gap requires more voltage than
the coil can reliably produce).
If you examine the newer autos, they typically will run gaps at .045+
(e.g., the GM direct ignition/distributor-less V6's). They also use
very high quality secondary circuit components. If you want to see a
relatively spectacular light show, pull a plug wire off one of the newer
GM (or most any maker's cars) engines at night, and watch the fire
works--very impressive. Oh, I should have said, pull the wire off and
then start it--pulling one off from a running engine can be, how should
we say, a rather electrifying experience.
*One caveat*, I have seen warnings about running the newer, ultra-high
voltage coils with no plug lead attached (a place for it to discharge)
as it could cause the coil to damage itself (too much voltage rise?).
It may not be a risk on the GM arrangement in that each coil has two
plugs attached (wasted spark configuration), but I have heard the
warning.
I replaced the spark plugs after 75K miles in a 90 Beretta GT (from
guilt, not need), and the gaps on the old plugs had worn to .065+ and
had passed the CA emissions test the previous week. This would indicate
that the GM coils will reliably throw 65K+ volts, though I did gap the
new plugs at .045).
One more consideration, coils require a certain amount of time to
saturate the secondary windings (they fire when you remove the current
to the primary circuit which causes the secondary circuit to
"collapse"--inducing the spark). Dwell is the number of degrees of
points cam rotation the points will remain closed (the coil is "on" and
saturating). We will see dwell times from 30 to 45 degrees or so, but
this is a very relative figure--the actual "time" the coil is saturating
depends on RPM--the slower the engine rotates, the longer the coil has
to saturate. So, unfortunately, the faster you spin your engine, the
less time your coil has to saturate between firings. What this leads us
to, is that your spark plug gap could be set wide (e.g., .045), and you
would notice that the engine started better and ran better from idle
through midrange RPM, but you may detect a missing at high RPM. This
could be caused by too much gap for your particular coil at that reduced
level of dwell (high RPM).
The short of it is that, if you are using points, you can run
practically any coil you wish as long as you you match impedence. A
good big coil from a GM V8 will work fine as long as you use an ignition
resistor. I use a coil from a Ford V8 in my BSA B50SS, and it throws
lightning bolts. I'm thinking about getting one of the new twin-lead GM
coils ($45) and modify the BSA for a twin plug head.
Anyway, if you are using a high-voltage coil with a small plug gap, you
are not getting the optimum spark from it, but don't over-gap for your
coil, particularly if you are running high RPM.
shook
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